1. Field of the Invention
The present invention relates to a nitride semiconductor device including an active layer of a multi quantum well structure, and more particularly, to a nitride semiconductor light emitting device including an active layer of a multi quantum well structure, in which a barrier layer adjacent to a p-type nitride semiconductor layer is Mg-doped into a p-doped barrier layer to prevent electrons from overflowing and accordingly improve hole injection efficiency, thereby enhancing external quantum efficiency.
2. Description of the Related Art
Recently, nitride semiconductor devices have been under development and have seen their applications broadened from a mobile phone keypad and a side view light source to general lightings and car headlights.
A light emitting diode (LED) used as a mobile phone utilizes an operating current of 20 mA. However, an LED for use in lightings and car headlights generally requires hundreds of mA of operating current to be supplied thereto. This supplied current of hundreds of mA entails problems which have been unobserved in a low current. A representative problem pertains to the efficiency droop.
In general, efficiency of the LED is represented by external quantum efficiency, which is a value obtained by multiplying photon extraction efficiency by internal quantum efficiency. Under the efficiency droop, the external quantum efficiency is decreased with an increase in the operating current.
In the conventional LED of a multi quantum well structure, efficiency droop occurs at a current density of about 10 A/cm2. That is, external quantum efficiency is gradually decreased at an operating current greater than the current density of 10 A/cm2. The current density is varied according to an area of the LED chip but the current density of 10 A/cm2 can be converted into the operating current of about 20 mA.
That is, as for the LED for use in a mobile phone, in the case of a current of 20 mA or more, a higher operating current leads to less efficiency of the LED. Therefore, the efficiency droop should be necessarily overcome to employ the LED in the lightings and car headlights requiring a high current density.
Specifically, in the conventional LED having a multi quantum well structure, the active layer includes an InGaN layer, i.e., well layer where light is emitted and a GaN layer, i.e., barrier layer. The well layer usually employs an undoped layer and the barrier layer employs an undoped layer or a Si-doped n-GaN layer.
The barrier layer is Si-doped to improve interface characteristics between the well layer and the barrier layer. Also, the Si-doping reduces resistance to decrease an operating voltage at a forward voltage. Moreover, the Si-doping increases concentration of electrons, thereby serving to screen a piezoelectric field caused by stress resulting from differences in lattice constant between the InGaN layer as the well layer and the GaN layer as the barrier layer.
This also reduces blue shift caused by an increase in the operating current. The screening effect ensures a reduction in band bending induced by piezoelectric field and allows for more overlapping of wave functions between the electrons and holes to enhance internal quantum efficiency.
This method has been used mostly to increase LED efficiency at a low current density. However, this does not prevent efficiency droop from occurring at a high current density as required by lightings and car headlights.
Furthermore, due to lower mobility of holes than electrons in general, the holes are injected into the active layer of the LED with low efficiency. On the contrary, the electrons are injected into the active layer well due to small effective mass and subsequently great mobility thereof.
Therefore, the holes are injected poorly while the electrons injected well migrate through the active layer over to the p-type nitride semiconductor layer. This phenomenon is aggravated at a high current density, i.e., by higher concentration of electrically injected carriers. This phenomenon leads to a decline in external quantum efficiency at a higher current density.
To improve this phenomenon, an electron blocking layer formed of AlGaN with great bandgap may be stacked on the active layer. This however does not prevent the electrons from overflowing since the electron block layer cannot serve as a barrier properly due to piezoelectric field arising from differences in lattice constant.
As described above, in the conventional LED having the multi quantum well structure, the holes are injected with low efficiency from a low current density to a high current density. Also, the external quantum efficiency is degraded due to nonradiative Auger recombination resulting from a high carrier concentration at a high current density.